The present invention relates to a method for preparing, through fermentation, L-isoleucine which is an amino acid essential for human and other animals and is principally useful as a material for various drugs represented by a medicine for promoting nutrition (nutrient), and an L-isoleucine-producing bacterium belonging to the genus Escherichia, which is used in the preparation method.
Isoleucine includes two asymmetric carbon atoms and therefore, it is difficult to industrially synthesize only the L-isomer thereof by a chemical synthetic method at a low price. Moreover, there have been known methods for preparing L-isoleucine which make use of biotransformation of precursors for biosynthesis of L-isoleucine such as .alpha.-aminobutyric acid and .alpha.-ketobutyric acid. However, these methods require the use of expensive raw materials and accordingly, are not useful methods for industrially preparing L-isoleucine at a low price.
The following have been known as methods for preparing L-isoleucine through direct fermentation using cheap carbon sources and nitrogen sources: 1) methods which employ, as L-isoleucine-producing strains, mutants belonging to the genus Corynebacterium, Serratia or Escherichia resistant to the antagonists against L-isoleucine (see, for instance, Japanese Examined Patent Publication (hereunder referred to as "J. P. KOKOKU") Nos. Sho 51-21077, Sho 52-4629, Sho 56-29998 and Sho 56-3035); and 2) methods which use, as L-isoleucine-producing strains, microorganisms belonging to the genus Escherichia or Corynebacterium, in which the function of threonine deaminase or acetohydroxylic acid synthase as a key enzyme for biosynthesis of L-isoleucine is enhanced by the recombinant DNA technology (see, for instance, Japanese Un-Examined Patent Publication (hereunder referred to as "J.P. KOKAI") Nos. Hei 2-458 and Hei 2-42988).